There are two problems with using late automobile units.
When you find one that looks to be about the right size, it will have the heater hoses go through the firewall directly behind your engine. The firewall will be flat instead of the concave area you have behind your engine.
When you find one with heater hoses on the passenger side, it will have a large section (suitcase) that has to be mounted on the outside as well as the unit that mounts inside.
I did it for my '67 which has more room than your '51 using a ford unit. It works great, but never again. Many, many more things had to be modified or fabricated than I had anticipated.
The compressor, condensor and charging the system is the easy part. You can source the wrecking yard for those items. I used a Sanden 508 from a VW and a parallel flow condensor from a '00 Chrysler. I had NAPA make up the hoses.
Next one will be an aftermarket type AC/Heat/Def unit.
